Best alternative word for "Basic building block" or like this
Solution 1:
Fundamental is described as:
1a : serving as a basis supporting existence or determining essential structure or function : BASIC
Elemental is another really good word, and is described by MW as:
b(1) : of, relating to, or being the basic or essential constituent of something : FUNDAMENTAL
Both are perfect in describing something rudimentary (which is another good word). You can use fundamental either as a noun, or as an adjective.
Source: https://www.merriam-webster.com/